The invention relates to a beam splitter for detecting the ratio of differently polarized components of a substantially monochromatic radiation.
Such beam splitters are required, for instance, in devices for stabilizing the frequency of lasers which oscillate in more than one mode, in order to be able to determine the intensity ratio of the modes present with different polarization, and use the intensity ratio to regulate resonator length in the laser.
Known devices for stabilizing the frequency of lasers, such as those described in German Pat. Nos. 2,043,734 and 2,053,246, so utilize the second output of the laser tube to be stabilized that the low-intensity and generally unused partial beam which there emerges is split into its differently polarized components by a polarization splitter, for example, a Wollaston prism. This second output of the laser tube is, however, not present or accessible in any tube available on the market.
To derive differently polarized components through coupling to the useful beam of the laser, it is also known from German Pat. No. 2,043,734 to arrange two Brewster plates, at an angle of 90.degree. apart, within the useful beam. Such a construction, however, has several disadvantages. On the one hand, the coupling factor, and thus the intensity of the derived partial bundle, is fixed, i.e., the intensity reduction in the useful beam which passes through may be too great. Furthermore, the parallel displacement of this beam by the Brewster plates results in adjustment problems. Finally, the plates must be very well coated to avoid reflection, so that multiple reflections cannot occur within the plate.
U.S. Pat. No. 3,588,738 describes a beam splitter for the above-mentioned purpose, consisting of a semi-transmitting mirror inclined at an angle of 45.degree. to the useful beam and a Wollaston prism arranged beyond the mirror. Such decoupling has the disadvantage that, due to phase shifts, linearly polarized light striking the semi-transmitting mirror layer is converted into elliptically polarized light. Furthermore, the intensity of the two decoupled partial beams which are polarized perpendicular to each other differs, depending on the angle which the surface of the splitter forms with the directions of polarization of the incident components.